Glaucoma affects about 70 million people worldwide, and is a disorder associated with high pressure in the eye resulting from the generation of excess intraocular fluid (aqueous humor). Aqueous humor is produced at a rate of 2-3 μl/min by the ciliary body and in a normal human eye maintains a constant intraocular pressure (“IOP”) around 12-20 mmHg. Aqueous humor exits the eye primarily through the trabecular meshwork and Schlemm's canal, where it eventually drains to the episcleral veins. Maintaining intraocular pressure within appropriate ranges is critical to health of the eye, and depends on aqueous humor dynamics, namely the production rate from the ciliary body (aqueous humor inflow) and its outflow rate through the trabeculum. The most frequent type of glaucoma, called open-angle glaucoma, results from an increase in the fluidic resistance of the trabecular meshwork. Left untreated, this disease typically causes damage to the optic nerve, with consequent loss of vision, initially peripheral, but progressively leading to total blindness. Unfortunately, glaucoma is often asymptomatic until late in the progress of the disease.
Traditionally, glaucoma is treated using medication, for example, the daily application of eye drops, such as Brinzolamide ophthalmic, that reduce production of aqueous humor. Such medications do not cure glaucoma, and must be continue to be taken to maintain intraocular pressures within accepted limits. In certain cases, such treatment may fail and other surgical treatments are employed, such as filter procedures or placement of a glaucoma drainage device. Glaucoma drainage devices reduce intraocular fluid pressure by providing an artificial drainage pathway, thus maintaining a low IOP.
Previously-known glaucoma drainage devices usually comprise a structure having a drainage tube that is inserted through a small incision made in the conjunctiva. The surgeon makes a tiny incision in the sclera of the eye and creates an opening for the drainage implant device. The drainage tube is placed such that the opening of the tube is disposed in the anterior chamber of the eye within the aqueous humor. The tube is sutured in place with the drainage device attached to the sclera of the eye. Many surgeons will place an absorbable suture around the tube at the time of surgery to prevent over-filtration through the device until a fibrous capsule has formed. Accordingly, such devices typically are not functional until about 3 to 8 weeks after the procedure, so as to prevent over-filtration.
An exemplary previously-known passive glaucoma drainage device is described in U.S. Pat. No. 4,457,757 to Molteno. The device described in that patent comprises a tube of a biologically inert silicone configured to be inserted into the eye to drain aqueous humor from the anterior chamber of the eye. The device does not include a pressure regulating mechanism, but instead relies on the resistance to aqueous flow through the tubing to prevent over drainage.
One drawback of devices such as those described in the Molteno patent is that the drainage flow depends on IOP and on the fixed hydrodynamic resistance of the shunt. In many cases, however, the hydrodynamic resistance of the shunt may not be sufficient to reduce high IOP when the resistance to flow is too high, or may lead to over-drainage if the resistance is low. For example, a common problem, which arises shortly after implantation, is hypotony, which occurs when IOP drops below acceptable physiological levels (i.e., IOP<10 mmHg). Hypotony usually takes place the first few days to weeks following the implantation of a glaucoma drainage device, and is a combined result of a low fluidic resistance of both the implant and the distal outflow paths. Hypotony may lead to a number of undesirable effects and complications, such as hypotensive maculopathy, cataract formation and optic nerve edema. Another problem, also related to the fixed fluid resistance of previously known implants, is fibrosis, which appears progressively at long term and which, depending on its extent and severity, may raise the effective fluidic resistance of the implant, thereby raising the TOP to different, often non-physiological, levels.
The foregoing drawbacks have been recognized in the prior art, and several improvements have been attempted to improve flow control over the entirely passive system described in Molteno.
For example, U.S. Pat. No. 5,411,473 to Ahmed describes a drainage device that includes a membrane-type valve. More specifically, Ahmed describes a drainage system including a membrane folded and held in tension between two plates to provide a slit opening, such that the membrane responds to pressure changes to open or close the slit opening. Unfortunately, the operational characteristics of the system depend on the properties of the membrane, which cannot be changed easily once the device is implanted. Also, the valve of Ahmed does not provide a true opening pressure to accurately control post-operation TOP.
U.S. Pat. No. 6,544,208 to Ethier describes a self-regulating pressure system. More specifically, Ethier describes an implantable shunt device having a flexible tube positioned in a pressurized enclosure. In this patent, flow through the tube is dependent on a differential pressure between a pressure in the flexible tube and a pressure outside the flexible tube in the pressurized enclosure. However, one skilled and experienced in the field of medical implants, especially in ophthalmology, would understand that such a system with a constant external pressure chamber would be very impractical, if not impossible, to make.
Ethier further describes that the pressure outside the flexible tube in the pressurized enclosure of the implantable shunt device is generated by osmotic effects. More specifically, the pressurized enclosure is filled with a solution containing a solute that generates an osmotic pressure which controls the opening pressure of the implantable shunt device. The implantable shunt device includes a semi-permeable membrane affixed between support gratings that reduce deformation of the semi-permeable membrane. Unfortunately, significant deformation of the semi-permeable membrane makes it difficult to predict the osmotic pressure within the pressurized enclosure.
U.S. Pat. No. 9,101,445 to Bigler describes an ocular drainage system for treating diseases that produce elevated intraocular pressures, such as glaucoma, wherein the system includes an implantable device and an external control unit. The implantable device includes a non-invasively adjustable valve featuring at least one deformable tube and a disk rotatably mounted within a housing, such that rotation of the disk using the external control unit causes the disk to apply a selected amount of compression to the deformable tube, thereby adjusting the fluidic resistance of the deformable tube and regulating the intraocular pressure.
Still other examples of previously-known systems are known. U.S. Pat. Nos. 5,626,558 and 6,508,779 to Suson describe a shunt which may be adjusted after implantation by using a low power laser to drill additional openings in the tube wall to adjust the flow rate. U.S. Pat. No. 6,186,974 to Allan et al. describes a drainage shunt having multiple layers, one of which may be a gel that swells upon absorption of fluid to adjust flow rate through the tube. U.S. Pat. No. 6,726,664 to Yaron describes a drainage tube including a distal hook that retains the distal end of the implant within the anterior chamber of the eye, and various means, such as rods or sutures, for partially occluding the lumen of the tube to regulate flow.
Other previously-known glaucoma treatment systems include significantly greater complexity to address the drawbacks of the simpler shunt systems described above. For example, U.S. Pat. No. 6,077,299 to Adelberg, et al. describes a non-invasively adjustable valve implant for the drainage of aqueous humor for treatment of glaucoma, wherein an implant having an inlet tube is surgically inserted in the anterior chamber of the eye to allow aqueous humor to flow from the anterior chamber to a valve. After passing through a pressure and/or flow regulating valve in the implant, the fluid is dispersed along the periphery of the implant to the interior of the Tenon's capsule where it is absorbed by the body. In one embodiment, the valve inhibits flow below, and allows flow above, a specific pressure difference between the TOP within the eye and the pressure within the bleb cavity in the Tenon's capsule. The specified pressure difference or set-point is always positive and the valve is always closed in the presence of negative pressure differences, to prevent reverse flow of fluid from the Tenon's capsule back into the anterior chamber of the eye.
In Adelberg, the valve is formed by a chamber to which the inlet tube is connected, such that the chamber is closed by a pressure sensitive valve in the shape of a flat cone. The pressure regulation set point of the valve is governed by a flexible diaphragm that cooperates with an armature plate having an inclined surface, and which is configured to slide over a complementary inclined surface attached to the diaphragm. Cooperation of the inclined surface of the plate and the complementary surface causes the diaphragm to deflect depending on where the armature plate is located. The armature plate is rotated, using a rotor and a set of speed-reducing and torque-enhancing gears, to regulate the flow through the device. The characteristics of the valve strongly depend on the configuration of the cone shaped valve. In addition, the regulating mechanism is complex, including many rotating parts and gears, and this complexity poses a risk of malfunction.
In view of the drawbacks of the foregoing prior art devices and methods, it would be desirable to provide an ocular drainage system and methods that are capable of maintaining a constant, or nearly constant, IOP.
It further would be desirable to provide an ocular drainage system effective to prevent hypotony post-implantation and/or effective in light of the development of fibrosis at long term.
It further would be desirable to provide an ocular drainage system having few moving parts, thereby enhancing robustness of the system and reducing the risk of failure arising from operation of complex mechanisms.
It further would be desirable to provide an ocular drainage system having a small volume to facilitate implantation of the device beneath the conjunctive, either under a relatively small scleral flap or on the scleral surface, or even within a diffuser plate.
It further would be desirable to provide an ocular drainage system wherein moving parts of the system are configured to reduce the risk of clogging or seizing due to the buildup of proteinaceous sediments.
Finally, it would be desirable to provide an ocular drainage system that permits the hydraulic resistance of the system to be dynamically adjusted in a non-invasive manner.